In automobile engines, lubricating oil is exposed to a blowby gas and heated for a long period of operation, so that the lubricating oil is contaminated with unburned hydrocarbons and degenerated oil additives (called “oil sludge” as a whole). The oil sludge includes oil sludge precursors having relatively low viscosity. The oil sludge attached to and accumulated on engine parts likely wears the parts and clogs lubricating oil paths, causing troubles in the functions of engine parts such as combined oil control rings (called “oil rings” unless otherwise mentioned). In the oil rings, spacer expanders and side rails are stuck to each other in the worst case, hindering the movement of separate side rails, and thus failing to exhibit a sufficient oil-controlling function.
As shown in FIG. 4(b), a conventional oil ring comprises a pair of annular side rails 120a, 120b each having a gap, and a spacer expander 101 supporting the side rails. As shown in FIG. 4(a), the spacer expander in an axially corrugated form has upper portions 102, lower portions 103, and leg portions 104 connecting the upper portions and the lower portions. The upper portions and the lower portions have tabs 105a, 105b on the inner side, and projections 106a, 106b supporting the side rails on the outer side, and dented intermediate portions 107a, 107b between the tabs and the projections. When the spacer expander is combined with the side rails, space 108a, 108b is defined by each tab, each projection, each intermediate portion and each side rail.
In the oil ring, the side rails 120a, 102b are pressed by radial and axial components of a force due to the inclination angle of the tabs 105a, 105b of the spacer expander 101, thereby sealing a cylinder wall surface and side surfaces of ring grooves of a piston. Particularly, a small-width oil ring having a small axial width h1 has good followability to a cylinder sidewall for a side-sealing function, resulting in reduced friction loss without increasing oil consumption even if it is a low-tension type. In the oil ring, however, oil sludge is easily accumulated in spaces 108a, 108b between the spacer expander and the side rails as described above. Particularly in the case of a small-width oil ring with extremely narrow space, the side rails 120a, 120b are highly likely stuck to the spacer expander 101 by the accumulated oil sludge. When sticking occurs, the followability of the side rails to the cylinder wall decreases extremely, resulting in drastically increased oil consumption.
To prevent the attachment and accumulation of oil sludge to an oil ring, the surface coatings of spacer expanders and side rails, and the structures of spacer expanders avoiding the accumulation of oil sludge were disclosed.
For example, Patent References 1 and 2 disclose a fluororesin coating and a fluororesin-containing resin coating, Patent Reference 3 discloses a coating containing fluoroalkyl-substituted alkoxide, Patent Reference 4 discloses a hydrophilic prepolymer coating containing inorganic polysilazane, Patent Reference 5 discloses a method for forming a metal coating having low surface free energy and hydrogen bonding force. They are water-repellant, oil-repellant coatings, or oppositely hydrophilic coatings, or those formed by methods of preventing the attachment of oil sludge.
As a structural measure, Patent Reference 6 discloses an oil ring comprising a spacer expander having upper and lower portions, and intermediate portions between the upper and lower portions, the intermediate portions having holes not reaching uprising portions of corrugation but sufficiently large to permit foreign materials such as lead compounds to pass. Patent References 7 and 8 disclose spacer expanders having structures comprising radial grooves in intermediate portions, and oil-exiting holes communicating with the grooves in tabs. Patent Reference 9 discloses a spacer expander having a structure preventing the accumulation of oil sludge in spaces between intermediate portions and side rails, the axial inclination angle θ of leg portions connecting upper and lower portions being 15° or more, and the upper and lower portions being circumferentially short.
However, the coating methods of Patent References 1-5 suffer excess steps leading to cost increase, and the hole-forming method of Patent Reference 6 needs difficult working, making the oil rings expensive. Also, oil resides in the grooves formed in intermediate portions in Patent References 7 and 8 while engines are stopped, likely suffering the accumulation of oil sludge. As a result, sufficient durability is not obtained in such an operation pattern as undergoing repeated start and stop of engines.